Photoelectron spectroscopy (abbreviated "PES") is used to measure the energy distribution of electrons that are ejected by light quanta from atoms or molecules of a specimen. The specimen is bombarded with X-rays or vacuum ultraviolet light of a known energy hv. Absorption of the radiation causes emission of electrons (termed "photoelectrons") with varying kinetic energy. These kinetic energies are analyzed to obtain a photoelectron spectrum of the specimen. PES performed using vacuum ultraviolet light (a technique abbreviated "UPS") is mainly used to study valence electrons, and PES performed using X-rays (X-ray photoelectron spectroscopy, abbreviated "XPS") is used to study internal (i.e., core) electrons of the specimen.
In conventional XPS apparatus, X-rays are generated from an X-ray tube in which Al or Mg is used as a cathode material. However, because of the low brightness of X-ray tubes and the short wavelength of X-rays, the spatial resolution of conventional XPS is limited to around 10 .mu.m. (Coxon et al., J. Electron Spectrosc. Relat. Phenom. 52:821 (1990). Recent trends in semiconductor manufacturing, materials science, and similar fields have greatly increased the demand for high spatial resolution XPS (abbreviated ".mu.-XPS") systems.
Quite high spatial resolution can be achieved using fine X-ray optics to converge the X-ray from, for example, the undulator radiation of a synchrotron facility. However, a synchrotron facility is huge and ordinary surface analysts do not have ready access to such an XPS system. Furthermore, synchrotron-based XPS systems do not allow for the in-situ observations that are very important for evaluation of semiconductor devices and materials. Therefore, a practical laboratory-sized .mu.-XPS system is strongly desired as an alternative to synchrotron-based XPS systems.
The concept of a laboratory-sized .mu.-XPS system employing a laser-plasma X-ray source and Time-of-Flight (abbreviated "TOF") analysis of photoelectron energy was first proposed by Tomie (U.S. Pat. No. 5,569,916).
X-rays are generated by irradiating a target material with pulsed laser light that produces a plasma at the target. Such a laser-produced X-ray source is termed a "Laser-Plasma X-ray Source" (abbreviated "LPX" source). An LPX source is a compact and highly brilliant X-ray source and is thus suitable for use as an X-ray source for .mu.-XPS.
In a TOF method of analysis, the energy of photoelectrons is determined from the time at which the photoelectrons arrive at the photoelectron detector and the distance from the specimen to the detector. Although the TOF method can be adopted only for a pulsed X-ray source, the efficiency with which such a method can detect photoelectrons can be many orders of magnitude greater than with other conventional methods.
Therefore, an XPS system employing LPX and TOF offers tantalizing prospects of providing a laboratory-sized .mu.-XPS apparatus exhibiting short data-acquisition times. Inventors Kondo, Tomie, and Shimizu first demonstrated that such an XPS could provide photoelectron spectra even after only a few laser shots. Advance Proceedings of the 42nd Conference of Applied Physics-Related Associations Lectures, p. 567, Mar. 29, 1995.
In a TOF method, in order to increase the energy resolution of detected photoelectrons, an 10 electrical field is generated inside a "flight tube." The electrical field, which extends parallel to the flight tube, serves to decrease the velocity of the photoelectrons and correspondingly increase the time required for the photoelectrons to reach the photoelectron detector. Kondo, Tomie, and Shimizu, Advance Proceedings of the 56th Applied Physics Society Conference Scientific Lectures, p. 494, Aug. 26, 1995).
As the distance between the specimen and the photoelectron detector is increased, according to conventional wisdom, in order to increase the energy resolution of the photoelectrons, the solid angle at which the photoelectrons are collected and detected by the detector becomes correspondingly smaller. This causes a decrease in the number of detected photoelectrons, which lowers the signal-to-noise (S/N) ratio of the apparatus. Also, whereas the velocity component of the photoelectrons in the direction of the flight-tube axis is decreased due to photoelectron divergence, the component in a direction perpendicular to the axis is unchanged. The electrical field also bends the trajectories of the photoelectrons so that some of the photoelectrons (that would otherwise reach the photoelectron detector if the electrical field were not present) no longer reach the detector.
As the electrical field strength is increased to improve the energy resolution of the detected photoelectrons, fewer photoelectrons actually reach the detector. This deteriorates the S/N ratio even further. In order to compensate for the decrease in number of photoelectrons, photoelectrons are guided to the detector along magnetic field lines by placing the specimen inside a divergent magnetic field and by trapping and collimating the photoelectrons emitted from the specimen surface in the magnetic field lines. (The divergent magnetic field is termed a "magnetic bottle.") See, e.g., Kruit and Read, J. Phys. E. 16:313 (1983). The photoelectron flux can be expanded while being guided to the detector along the magnetic field lines, and the trajectory direction of the photoelectrons can be held nearly parallel to the axis of the flight tube. According to the thinking behind such apparatus, by applying such a "retarding field," there is no decrease in the number of detected photoelectrons since most of the photoelectrons are collimated in the flight tube. However, the magnification factor in the lateral direction resulting from application of such a divergent magnetic field is at most approximately 100.times.. E.g., if X-rays are converged onto a 1-.mu.m locus on the specimen, the diameter of the photoelectron flux on the detector would be no more than approximately 100 .mu.m. If a microchannel plate (MCP) were used as the photoelectron detector, photoelectrons would enter no more than a mere fifteen or so microchannels of the MCP.
On the other hand, if the number of photoelectrons entering such a small number of microchannels per unit time were to be increased, then a large current would flow through the affected microchannels. This would make the MCP susceptible to problems such as deteriorated response time, gain fluctuations, and damage, resulting in decreased MCP performance. Large numbers of electrons entering a microchannel on the detection surface per unit time in such a way causes substantial problems.